Dynamic magnetic recording techniques have advanced to the state of attaining very reliable and compact records for digital data. As a result, binary data records are widely used on magnetic tapes, discs, strips and so on. Recognizing the wide variety of single-track, self-clocking formats for magnetically recording information, one recording format in general use has been variously referred to as F/2F recording, also known as Aiken F/2F, which is one of a class of codes known as Manchester codes. That format records both binary code data and timing information (clock or sync signals) in a single magnetic track. Essentially, the sync pulses are recorded at regular intervals by a magnetic discontinuity in the form of contiguous magnetic poles, i.e. either N--N or S--S. So recorded, the sync pulses define spaces therebetween for receiving either a binary data pulse (indicative of a binary "one") or no pulse (indicative of a binary "zero"). The binary data pulses are also recorded as magnetic discontinuities. Recorded pulses must alternatively be north and south poles to accommodate stable magnetic domains. Accordingly, both sync pulses and binary data pulses may be represented by abutting poles of either polarity, discrimination between sync and binary being accomplished by time tracking techniques.
In recording binary information in accordance with the above format, it is conventional practice to record a preliminary code in the track to initiate the time tracking operation for distinguishing between sync pulses and binary data pulse signals. The difficulty of maintaining proper time tracking (to segregate sync and data pulses) is related to the quality of the magnetic record and the precision with which it is moved in relation to the transducer head, e.g. uniformity of speed and spacing between head and medium. In advanced technology equipment, as computer peripheral magnetic tape units utilizing current state of the art, little difficulty would be expected to be encountered. However, a substantial number of magnetic recording applications exist which involve considerable difficulty in this regard. For example, magnetic strip credit cards carry a recording medium which must be expected to experience considerable adverse treatment. Furthermore, it is sometimes desirable to sense or "read" the magnetic stripes of such cards using a manual apparatus, e.g. handheld and manually moved unit. Such an apparatus illustrates the need for a magnetic sensing system with considerable tolerance for speed variations and record deterioration. In general, the present invention affords reliable magnetic sensing and signal processing in applications involving adverse conditions.
The disclosed embodiment of the present invention utilizes amplitude discrimination to reject spurious signals, the threshold level being established in accordance with the strength (amplitude) of prior signals and the character of prior binary data. In F/2F data recording, if a prior binary bit was manifest by a pulse, e.g. conventionally a "one", then the pulse density of the record is increased which reflects upon the signal amplitude as well known in the prior art, and in accordance with the present invention is compensated correctively. Pulse density also reflects on the position (time and space related) at which recorded data is observed. In the present system, time tracking to distinguish sync and binary data pulses reflects the character of a prior binary data pulse signal as an element of control.